Sansui 5000a FM muting and stereo issue

[Logchop]

Hello,

Have a 5000a here with some FM issues, a pretty common thing with this model it seems. I've searched extensively on this site to see if anyone had the exact same problem I'm running into, but haven't seen it. Currently, I'm not able to get any FM in stereo and the sound through FM is muted, as if the muting button is always pressed even for stations that are tuned in. I have confirmed that the stereo light is not burnt out and any audio coming through in FM seems to be in mono, even on strong stations that show good reception on the signal meter. The signal meter and tuning meter work fine. All pots have been cleaned with deoxit including the level pot on the back of the unit and the 75-300 ohm antenna switch.

I've read that mpl251 on the F-1120 board is a common culprit for the FM input being muted, but I've opened up my unit and can't seem to find the F-1120 board. Pictures attached. I was under the impression that the F-1120 board should be located on the underside of the unit between the F-1006a and F-1219 boards. I'm not sure if I'm missing something or if the receiver was altered at some point, it does have a sticker on top of the case saying it was repaired 8-20-87. Unfortunately, that repair did not include updating the F-1040-1 boards, which I also intend on doing following Skippy's method.

I'd appreciate any help finding this F-1120 board, figuring out the FM muting, and figuring out why FM isn't coming through in stereo. Thanks!

 

[AE Bob]

I have seen several 5000's where the stereo indicator would not light. The reason was an open diode (go figure) on the small power supply regulator board under the chassis (the one with the two gray capacitors). It is the diode to the left of the larger capacitor in your second photo. That supply only feeds the stereo indicator lamp. From what I can see in my 5000A schematic the muting is done differently than the original 5000. It is performed at the end of the IF amplifier. The signal meter is fed from the output of the second IF stage, so a defect in the third or last stage could cause audio issues as well as no stereo, with a good signal meter reading. I would strongly suspect one of the IF amp IC's is defective. They are UA703's or equivalents. They commonly fail. If you have a scope, you can look at the IF signal through each stage. There should be a good deal of gain in each stage. If one of the IF IC's is bad you will see no gain or a loss in signal level in that stage.

 

[Logchop]

I have seen several 5000's where the stereo indicator would not light. The reason was an open diode (go figure) on the small power supply regulator board under the chassis (the one with the two gray capacitors). It is the diode to the left of the larger capacitor in your second photo. That supply only feeds the stereo indicator lamp. From what I can see in my 5000A schematic the muting is done differently than the original 5000. It is performed at the end of the IF amplifier. The signal meter is fed from the output of the second IF stage, so a defect in the third or last stage could cause audio issues as well as no stereo, with a good signal meter reading. I would strongly suspect one of the IF amp IC's is defective. They are UA703's or equivalents. They commonly fail. If you have a scope, you can look at the IF signal through each stage. There should be a good deal of gain in each stage. If one of the IF IC's is bad you will see no gain or a loss in signal level in that stage.

This is great advice! Thank you, Bob. I don't have a ton of experience with oscopes but I do have one on hand. It is only 10 MHz though, and I have read that that may not be sufficient when looking at FM boards. When you refer to the IF stage ICs, do you mean IC201-204 on the F-1219 board? I believe all four are labeled PA7703. I want to make sure I'm understanding what you're saying and don't mess around where I don't need to be. I'm thinking I'll look at those ICs first to see if they're the culprit, and then look at the diode as that's an easier fix. I do suspect that the stereo light not coming on is related to the muted FM output, unless I'm unlucky and that diode has also opened up.

When it comes to the FM muting, is it fair to say that some of the 5000a units use the F-1120 muting board while others don't? I have seen other pictures of 5000a units with the 1120 board, but unless I'm completely blind I'm not seeing it in mine.

Thanks for any more help you can offer. People like you are the reason I subscribed to AK, there's a great wealth of knowledge here. Hopefully at some point I'll have enough experience to chip in and help others as well.

 

[kwayde]

This is great advice! Thank you, Bob. I don't have a ton of experience with oscopes but I do have one on hand. It is only 10 MHz though, and I have read that that may not be sufficient when looking at FM boards. When you refer to the IF stage ICs, do you mean IC201-204 on the F-1219 board? I believe all four are labeled PA7703. I want to make sure I'm understanding what you're saying and don't mess around where I don't need to be. I'm thinking I'll look at those ICs first to see if they're the culprit, and then look at the diode as that's an easier fix. I do suspect that the stereo light not coming on is related to the muted FM output, unless I'm unlucky and that diode has also opened up.

When it comes to the FM muting, is it fair to say that some of the 5000a units use the F-1120 muting board while others don't? I have seen other pictures of 5000a units with the 1120 board, but unless I'm completely blind I'm not seeing it in mine.

Thanks for any more help you can offer. People like you are the reason I subscribed to AK, there's a great wealth of knowledge here. Hopefully at some point I'll have enough experience to chip in and help others as well.

Apparently yours doesn't have the F-1120 board.

Here's a pic of the 5000A I recently worked on with the board circled in yellow.

 

[kwayde]

Also, when I was looking through my downloads I found two different schematics for the 5000A. One is shown as a M-5000A that has a smaller F-1120 board with only one transistor on it which connects to a F-1219 board that is similar to the F-1005 until it gets towards the outputs.

 

[Logchop]

Also, when I was looking through my downloads I found two different schematics for the 5000A. One is shown as a M-5000A that has a smaller F-1120 board with only one transistor on it which connects to a F-1219 board that is similar to the F-1005 until it gets towards the outputs.

View attachment 3226524


View attachment 3226525

This is very interesting. I did know that there were two versions of the F-1120 board, one with less components on it. But now I guess we know that there's a third 5000a variation with no F-1120 board. The schematics that were in the folder with mine show the 1120 board, which led to my confusion. I wonder if the different variations depend on the date/year of production. I also wonder exactly how mine differs from the others in terms of FM muting without the 1120 board. Must have something to do with the IF amp like Bob mentioned.

 

[AE Bob]

You can check if the diode is open by simply measuring the voltage on the cathode. I forget what the exact voltage is , but it is somewhere around 6 to 12 volts. If it is missing the diode is open. I strongly suspect you have a bad IF IC. They may have different part numbers on them, but they are all the same part. Some are in a metal can and some are a ceramic base with an epoxy top. They may have numbers like IC7, TRO9012, UA703, as well as several others. I would look at each stage with your scope, and see if the signal disappears (or gets very low) after one of the last two IC's in the IF amp. The last two IC's will not effect the signal meter, as the source for it comes off the second IF amplifier stage.

 

[Guido57]

What is the serial number on the unit? Perhaps give us an idea of where it fell in the production run.

 

[Logchop]

You can check if the diode is open by simply measuring the voltage on the cathode. I forget what the exact voltage is , but it is somewhere around 6 to 12 volts. If it is missing the diode is open. I strongly suspect you have a bad IF IC. They may have different part numbers on them, but they are all the same part. Some are in a metal can and some are a ceramic base with an epoxy top. They may have numbers like IC7, TRO9012, UA703, as well as several others. I would look at each stage with your scope, and see if the signal disappears (or gets very low) after one of the last two IC's in the IF amp. The last two IC's will not effect the signal meter, as the source for it comes off the second IF amplifier stage.

Thanks Bob, will get to this when I can and report back. I'll let you know if I have more questions.

What is the serial number on the unit? Perhaps give us an idea of where it fell in the production run.

210021266